For a better understanding of this invention, several terms should be defined. The term "electrode" means a conductive device which is capable of transmitting an electrical signal. The term "nerve" describes a structure of fibers arranged in bundles held together by a connective tissue sheath. Stimuli are transmitted from the central nervous system to the periphery or the reverse through the fibers. The term "neural" means any structure that is composed of nerve cells or their components, or that on further development will give rise to nerve cells. The term "neurite" describes the filamentous neural projections that grow from nerves, usually from the nerve axon which normally conduct nerve impulses away from the cell body and the dendrites. The term "sciatic nerve" refers to the long nerve of the leg that extends from the back to the tip of the toes and carries both motor and sensory fiber. The term "cortex" describes the outer portion of an organ such as the brain as distinguished from the inner, or medullary, portion. The cerebral cortex is the layer of grey matter (1-4 mm thick) covering the entire outer surface of the cerebral hemisphere of mammals.
Muscle paralysis affects almost 100,000 quadriplegics in the United States and approximately 1 million persons worldwide. The optimum goal of the quadriplegic's rehabilitation is return of movement by activation of paralyzed muscles under direct control of the central nervous system. This has been achieved to some extent by functional neuromuscular stimulation. Using shoulder movements as control signals, the muscles of the paralyzed hand can be electrically stimulated to grasp and release. The problem remains to obtain better control signals because (1) shoulder control signals are insufficiently precise for the force and velocity control required by delicate movements, and (2) restoration of walking and complex forearm/hand movements and following spinal cord injury require multiple control signals. Precise, multiple control signals are found only within the central nervous system. Thus, to obtain information on the signals required for central nervous system control of reactivated muscles, there is a need to record a reliable usable signal over long periods such as months or years.
An even greater need in terms of numbers exists for a motor control device which would be beneficial for persons other than those with spinal cord injuries. Recording from cortical areas (other than area 4 of the cortex, as defined by current cortical mapping methods) such as the supplementary motor areas ("SMA") and premotor areas ("PMA") would be possible in patients who have a hemiparesis due to a stroke affecting the cortex supplied by the middle cerebral artery. In these lesions, the hemiparesis can be due to infarction of area 4 motor cortex and post-central cortex, but more rostral areas containing SMA and PMA are normally spared because they are supplied by the anterior cerebral artery. Recording from these areas in a stabilized stroke patient may be feasible because of the phenomenon of conditioning.
Conditioning of neuronal firing patterns has been demonstrated in performing monkeys, where monkeys were rewarded for activating neurons at particular firing rates, or activating particular neurons. The monkeys even succeeded in activating one neuron and suppressing the activity of another, the activity of both neurons being recorded simultaneously through the same electrode. This not only suggests that the desired neurons can be conditioned to fire at specific rates, but that undesirable neurons can be suppressed if contribution to unwanted background activity, thus physiologically improving the signal-to-noise ratio.
Conditioning of neuronal activity is even more likely to be successful in the strongly motivated human. Scientists have shown that the supplementary motor area of humans is under voluntary control. Subjects were asked to think about moving a finger but not actually move it. Cerebral blood flow (an indicator of neuronal activity) increased in the SMA but not in area 4. These studies in the human, plus the studies at the single cell level in sub-human primates, imply a high probability that humans can condition the activity of individual neurons.
There is yet another significant feature about recording multiple neurons: a wealth of basic neurophysiological data could become available. Questions regarding the functional organization of adjacent neurons, their relative activities during sensory perception, and their relative coordinated activities during motor output could be determined. Relative activity during conditioning and during learning of new tasks could be studied. Furthermore, implanting electrodes over different cortical areas, for example, area 4 and the SMA, could demonstrate functional interactions in a manner unavailable by any other means.
Efforts to implant electrodes have met with some success, but still encounter many problems. Typically, a wire or wires is implanted in the cortex, the wire is immobilized on the skull in some manner and connected to an amplification and recording device. These methods and devices are deficient in that movement of the electrode within the skull can disrupt signal transmission or cause signal artifacts. Nonflexibility of the electrode can cause, in addition to signal disruption, damage to the cortex. Moreover, there is the possibility of local tissue reaction to the presence of a foreign body or scar tissue formation over time which can decrease the usefulness of the electrode and the signal transmitted. Infection due to electrode wires can cause deleterious effects. Current implant electrodes have been used to record signals over a period of day or weeks, and in few instances, for several months. An electrode is needed, however, that can transmit signals accurately over a longer period, since repeated operations on a human to repair or replace an electrode are certainly undesirable. Freedom of movement is also often restricted by the bulky electrode structures used by present techniques. It would be advantageous to have a small structure that would not limit movement. A permanent electrode, along with a recording system which does not restrict movement would allow the gathering of data which, eventually, could enable paralyzed persons to interface with prosthetic devices in a way approaching normal motor functions, that is, with accuracy and precision.